1. Field of the Invention
The present invention relates to technology for amplifying audio signals of a plurality of channels.
2. Description of the Related Art
In order to realize sound effects having a 3-dimensional sensation, audio signals are being used with multi-channels, and the number of speakers used is dramatically increasing. For example, 5.1 channels, which have become mainstream in audio systems in the movie world, have also been applied in recent years to DVD video, digital broadcasting, car audio, and the like. The 5.1 channels are based on 6 channels: front left, front right, rear left, rear right, center, and sub-woofer. In the car audio field, there are cases in which a person in a front seat listens to car navigation audio, while a person in a rear seat listens to audio from a DVD player, and from this type of trend also, the number of speakers is increasing.
With regard to technology related to amplification of audio signals, there has been a proposal with an aim of reducing noise when volume is switched (Patent Document 1). Patent Document 1 discloses technology for reducing shock sound when volume is switched by gradually changing volume control output.
Patent Document 1: Japanese Patent Application, Laid Open No. 2002-26670
Patent Document 2: Japanese Patent Application, Laid Open No. 2005-286532
When considering switching the volume in an acoustic system for multi-channel audio signals, the inventors of this invention recognized the following types of problems.
As an example, when volume data for each channel is transmitted from a microcomputer in accordance with a volume switching instruction from a user, from a constraint of a bus connecting the microcomputer and a volume control side, the volume data of each channel is received at the volume control side with an accompanying time lag. In such cases, a time lag occurs also in switching the volume for each channel, and the user feels something is wrong. This becomes more significant as the number of channels increases.
Countermeasures that may be considered include enlarging bus width of a bus connecting the microcomputer and the volume control side, or, with the same bus widths, using one which has a fast transfer speed. However, enlarging the bus width leads to an increase in the number of pins, and increasing the transfer speed leads to an increase in power consumption. Furthermore, these countermeasures may impede usage of a standard bus. Therefore, it has to be stated that in the present situation getting rid of the time lag in reception of the volume data is unrealistic.
Furthermore, in cases of using technology for countermeasures to switching noise, as disclosed in Patent Document 1, if a configuration for making the change of volume control output gradual is commonly used by a plurality of channels due to constraints of circuit scale, it is no longer possible to change the volume of other channels while the volume control output is being gradually changed for one channel. In such cases, the feeling of the user that something is wrong, due to the time lag, becomes more serious. For example, in 5.1 channel systems, if the volume is being switched in a sequence of center, front right, rear right, rear left, and front left, the user may have a sense that the audio is revolving.